Postreplication gaps formed at pyrimidine dimers during DNA replication in ultraviolet-irradiated cells are known to cause sister exchanges. Exchanges are also caused, even in the absence of DNA replication, by cross-links in DNA molecules, but only in excision-proficient cells. It is planned to investigate how postreplication gaps and incised cross-links in DNA molecules initiate genetic exchanges. In preparation for studies with an in vitro system, extracts of E. coli comparable to those used for replication of phage phiX, will be prepared. Adding appropriate nucleoside triphosphates, control and UV-irradiated phiX viral DNA will be replicated. PhiX RF I DNA will be prepared and cross-linked. Studies will be made of the stability of phiX DNA in these cell extracts, their ability to produce partially replicated viral DNA, and their ability to incise cross-linked duplex molecules. We next plan to study the interaction between these damaged molecules and intact covalent circular phiX DNA, looking for evidence of radioactive transfer to the fast sedimenting form as an indication of in vitro recombination. BIBLIOGRAPHIC REFERENCES: Ljungquist, S., Lindahl, T. and P. Howard-Flanders (1976). Methyl methane sulfonate-sensitive mutant of Escherichia coli deficient in an endonuclease specific for apurinic sites in deoxyribonucleic acid. J. Bacteriol. 126, 646-653. Lin, P.-F. and P. Howard-Flanders (1976). Genetic exchanges caused by ultra-violet photoproducts in phage lambda DNA molecules: The role of DNA replication. Molec. gen. Genet. 146, 107-115.